CAIRO — The U.S. increased pressure Friday on Egypt’s military rulers to hand over power to civilian leaders, and the generals turned to a Mubarak-era politician to head a new government in a move that failed to satisfy the more than 100,000 protesters who jammed Tahrir Square in the biggest rally yet this week.

The demonstrators rejected the appointment of Kamal el-Ganzouri as prime minister, breaking into chants of “Illegitimate! Illegitimate!” and setting up a showdown between the two sides only three days before key parliamentary elections.

The size of the rally and the resilience of protesters in the face of the violence used by security forces in this week’s deadly street battles have won back for the movement much of the strength it projected during the 18-day uprising that ousted President Hosni Mubarak in February.

Showing the sort of resolve from the earliest days of the Arab Spring, the protesters say they will not leave the iconic square until the military rulers led by Field Marshal Hussein Tantawi step down and a civilian presidential council is formed to run the country until a new leader is elected.

“They stole our January revolution because we did not agree on who should represent us,” said activist Sedeeqah Abu Seadah. “We shouted ‘erhal’ (leave) but did not shout the name of the person we want.”

The military’s appointment of el-Ganzouri, its apology for the death of protesters and a series of partial concessions in the past two days suggest that the generals are struggling to overcome the most serious challenge to their nine-month rule, with fewer options now available to them.

Significantly adding to their predicament, the Obama administration brought its position on the crisis in Egypt closer to the protesters’ demands, urging the military to fully empower the next interim civilian government.

“We believe that Egypt’s transition to democracy must continue, with elections proceeding expeditiously, and all necessary measures taken to ensure security and prevent intimidation,” the White House said in a statement.

“Most importantly, we believe that the full transfer of power to a civilian government must take place in a just and inclusive manner that responds to the legitimate aspirations of the Egyptian people, as soon as possible,” it said.

The adjustment in the Obama administration’s approach is significant because the Egyptian military, the nation's most powerful institution, has in the past 30 years forged close relations with successive U.S. administrations, receiving $1.3 billion annually in aid. It followed the public U.S. endorsement of the military’s original timetable for the transfer of power by late 2012 or early 2013.

The military inadvertently sparked the ongoing unrest by pushing plans for a political “guardianship” role for itself and immunity from civilian oversight even after a new parliament is seated and a new president is elected.

The last straw came when the military ordered the use of force against a small protest in Tahrir Square last weekend and then launched a failed, joint army-police raid to evacuate a larger crowd. Nearly 40 protesters have died in the past week.

The latest crisis has overshadowed Monday’s start of Egypt’s first parliamentary elections since Mubarak was replaced by Tantawi. The vote, which the generals say will be held on schedule despite the unrest, is now seen by many activists and protesters to be serving the military’s efforts to project an image of itself as the nation’s saviors and true democrats.

The next parliament is expected to be dominated by Islamists, whose political groups have decided to boycott the ongoing protests to keep from doing anything that could derail the election. However, the outcome of the vote is likely to be seen as flawed given the growing unrest and the suspension by many candidates of their campaigns in solidarity with the protesters.

The Social Democrats, a political party born out of the January-February uprising, said Friday that it would boycott the election, arguing that the vote would not be fair given the tension caused by the unrest, which it blamed on the military.

In rejecting el-Ganzouri’s government, 24 protest groups, including two political parties, announced the formation of their own “national salvation” government that they say represents the protesters. The group will be headed by Nobel peace laureate Mohamed ElBaradei and four deputies.

While it is unclear how many people the statement announcing the move represents, it is an attempt by the protest movement to unify its demands and leadership.

Supporters of the military staged a rival demonstration Friday across town from Tahrir, but only several thousand people turned out. They waved identical, brand new Egyptian flags that prompted activists to post on social network sites their suspicion that the demonstration may have been staged by the military.

State television, whose coverage of the crisis shows a clear, pro-military bias, gave prominence to the supporters of the generals and hosted commentators discrediting the Tahrir protesters as irresponsible youths and violent football hooligans.

“El-Ganzouri is over and done with. We want young people to take charge of the country,” said Hamdi Arban, a 50-year-old lawyer who was in Tahrir Square. “We will stay here and we won’t get our rights except from here,” he said.

Basma el-Husseini, who directs a cultural center and was also in Tahrir, dismissed the 78-year-old el-Ganzouri as a man with little energy to keep up with the multitude of challenges facing Egypt. “They (the generals) don’t get the power of the people. All they are doing now is play for time to make people fed up.”

El-Ganzouri served as prime minister between 1996 and 1999 and was deputy prime minister and planning minister before that. Tantawi himself served under el-Ganzouri for three of the 20 years he spent as Mubarak’s defense minister.

Addressing a televised news conference, el-Ganzouri said the military has given him greater powers than his predecessor and that he wouldn’t have accepted the job if he believed Tantawi had any intention of staying in power.

“The powers given to me exceed any similar mandates,” he said. “I will take full authority so I’m able to serve my country.”

But el-Ganzouri appeared uncomfortable, grasping for words and repeatedly pausing as he spoke, giving rambling answers when pressed whether he could form a government that will satisfy the public when many prominent figures have shunned joining the new administration.

The choice of el-Ganzouri deepened the anger of the protesters, already seething over the military’s perceived reluctance to dismantle the legacy of Mubarak’s 29-year rule.

El-Ganzouri replaces Essam Sharaf, who resigned this week after nearly nine months in office. Sharaf was criticized for being weak and beholden to the generals.

Friday’s protest in Tahrir was dubbed by organizers as “The Last Chance Million-Man Protest.” Swelling crowds chanted, “Leave! Leave!” and “The people want to bring down the field marshal!”

ElBaradei was mobbed by hundreds of supporters as he arrived in the square and took part in Friday prayers, leaving shortly afterward. Some factions in the protest have cited ElBaradei as a possible member of a civilian presidential council they want to replace the generals.

“He is here to support the revolutionaries,” said protester Ahmed Awad, 35. “He came to see for himself the tragedy caused by the military.”

Fireworks lit the sky in the evening and a large banner was strung over a side street called Mohammed Mahmoud, where most of the fighting occurred, declaring that it would be renamed “Eyes of the Revolution Street,” in honor of the hundreds of protesters who suffered eye injuries as a result of tear gas used by police.

About 500 protesters camped out in front of the Cabinet office, vowing to remain to prevent el-Ganzouri’s government from entering the building.

Thousands of pro-democracy protesters also rallied in other cities, including at least 10,000 in Alexandria and smaller crowds in Luxor and Assiut in southern Egypt.

The military has rejected calls to immediately step down, saying its claim to power is supported by the warm welcome given to troops who took over the streets from the discredited police early in the anti-Mubarak uprising as well as an overwhelming endorsement for constitutional amendments they proposed in a March referendum.

Tantawi has offered another referendum on whether his military council should step down immediately.

Such a vote, activists say, would be divisive and likely open the door for a deal between the military and political groups, particularly the Muslim Brotherhood. Egypt’s largest and best organized group, the Brotherhood is notorious for its opportunism and thirst for power. It was empowered after the fall of Mubarak, regaining legitimacy after spending nearly 60 years as an outlawed group.